1. Field of the Invention
The present invention relates to a fastener driving tool for striking a nail to insert the nail into a material such as wood using a power source of compressed air. More particularly, the present invention relates to a fastener driving tool for striking a nail including technology of holding a piston at an upper dead center and cooling a bumper.
2. Description of the Related Art
A bumper generates heat by absorbing extra energy of a piston in striking. In recent years, it is requested to promote durability of a bumper in accordance with high output formation of a fastener driving tool.
There is a description in JP-A-2003-236768 as follows. By installing an exhaust path for exhausting compressed air to the atmosphere after striking a fastener to be brought into contact with an outer peripheral portion of a cylinder, compressed air a temperature of which is dropped by being adiabatically expanded in the exhaust path cools the cylinder.
According to JP-A-2002-321168, by providing a hole for communicating a return air chamber and inside of a cylinder by constituting an angle in a horizontal direction relative to a radius direction of the cylinder, a swirl flow is produced at an outer periphery of a bumper to cool the bumper.
Further, there is a fastener driving tool as shown by FIG. 10 and FIG. 11. The fastener driving tool as illustrated in FIGS. 10 and 11 includes in a portion thereof a piston 5, an O ring 7 and a cylinder 6. Further, technical terms and notations of respective portions are commonly used.
An outer diameter of the O ring 7 is designed to be slightly larger than an inner diameter of a small diameter portion 6a of the cylinder 6 and to produce a clearance between an inner diameter of the O ring 7 and an outer diameter of a groove 5a of the piston 5. As shown by FIG. 10, by making compressed air from a returning chamber flow, the O ring 7 is pressed in an outer diameter direction as shown by arrow marks to seal at point A and point C. A total of the O ring 7 is exerted with a force as shown by an arrow mark D directed from a center O of the O ring 7 to a middle point of point A and point C.
The cylinder 6 is formed with a large diameter portion 6c at an upper end thereof, and an inclined face 6b is extended from the large diameter portion 6c in a lower direction to connect to the small diameter portion 6a. A clearance between the large diameter portion 6c and the O ring 7 is about 2.4 mm in a diameter direction. When the piston 5 is disposed at a position shown in FIG. 10, the piston 5 and the small diameter portion 6a are brought into contact with each other to form the seal. When the piston 5 is disposed at a position shown in FIG. 11, a clearance is produced between the piston 5 and the large diameter portion 6c. 
Therefore, when the piston 5 is returned to a side of the upper dead center by compressed air from a lower side of the piston 5 after striking operation, in the case in which the piston 5 is disposed at the position shown in FIG. 10, compressed air is not leaked from between the piston 5 and the cylinder 6 and all of compressed air is exhausted from a clearance by way of a bumper provided at the lower dead center of the piston 5. When the piston 5 returns to a position shown in FIG. 11, that is, a vicinity of the upper dead center, compressed air is exhausted also from a clearance between the piston 5 and the cylinder 6 and therefore, a flow of air impinging on the bumper is reduced.
The constitution described in JP-A-2003-236768 and JP-A-2002-321168 is difficult to fabricate and difficult to realize. Further, according to the fastener driving tool shown in FIG. 10 and FIG. 11, cooling of the bumper is insufficient.